SiGe FINFET WITH IMPROVED JUNCTION DOPING CONTROL

ABSTRACT

A semiconductor device and a method for fabricating the device. The method includes: providing a FinFET having a source/drain region, at least one SiGe fin, a silicon substrate, a local oxide layer is formed on the silicon substrate, a gate structure is formed on the at least one SiGe fin and the local oxide layer, the gate structure is encapsulated by a gate hard mask and sidewall spacer layers; recessing the at least one SiGe fin in the source/drain region to the sidewall spacer layers and the silicon substrate layer; recessing the local oxide layer in the source/drain region to the sidewall spacer layer and the silicon substrate; growing a n-doped silicon layer on the silicon substrate; growing a p-doped silicon layer or p-doped SiGe layer on the n-doped silicon layer; and forming a silicide layer on the p-doped silicon layer or p-doped SiGe layer.

DOMESTIC PRIORITY

This application claims priority to United States ProvisionalApplication No. 61/976,012, entitled “SiGe FINFET WITH IMPROVED JUNCTIONDOPING CONTROL,” filed Apr. 7, 2014, which is incorporated herein byreference in its entirety.

BACKGROUND

The present invention relates to the field of semiconductor devices.More particularly, the present invention is related to a device andmethod for forming an extension in a FinFET device.

FinFET (Fin field-effect transistor) is a technology that allows forhigher performance in smaller devices. A bulk FinFET structure includesa fin, a gate, and a source drain region. The structure has narrow barsof SiGe (silicon-germanium) fins with a gate or gates on either side ofthe fins. The gates can also be located on top of the fins. SiGe FinFETdevices are designed to help reduce the threshold voltage and improvedevice performance. The source/drain region of the MOSFET (metal oxidesemiconductor field-effect transistor) is doped by adding trace impurityelements into the substrate which allows the electrical properties to bealtered. The source/drain region can be doped by either ion implantationor by merging the source/drain regions with an epitaxy layer and dopingthis epitaxy layer in-situ. After doping is complete, the bulk wafer isannealed for diffusion and activation of the dopant to form thesource/drain and extension junctions. Doping with ion implantationcauses fin damage and incomplete or no recrystallization upon annealing.This leads to defects and incomplete activation of the dopant, which inturn leads to higher leakage and higher access resistance, respectively.Boron is the common dopant for positive channel field effect transistor(pFET) devices. Diffusion of boron from merged epitaxy layer into theSiGe fins is extremely slow and can leave a significant portion of theSiGe fin undoped, thus leading to underlapped devices with very highaccess resistance. SiGe FinFET devices with overlapped junction andlower access resistance are a challenge as boron diffusion in SiGe isnegligible.

SUMMARY

According to one aspect of the present invention, a method forfabricating a semiconductor device is provided. The method includes:providing a FinFET having a source/drain region and at least one SiGefin, wherein the at least one SiGe fin has sidewalls and is formed on asilicon substrate, a local oxide layer is formed on the siliconsubstrate, a gate structure is formed on the at least one SiGe fin andthe local oxide layer, the gate structure is encapsulated by a gate hardmask and a sidewall spacer layer; recessing the at least one SiGe fin inthe source/drain region, wherein the at least one SiGe fin is recessedto the sidewall spacer layer and the silicon substrate layer; recessingthe local oxide layer in the source/drain region, wherein the localoxide layer is recessed to the sidewall spacer layer and the siliconsubstrate; growing a n-doped silicon layer on the silicon substrate inthe source/drain region; growing either a p-doped silicon layer or ap-doped SiGe layer on the n-doped silicon layer in the source/drainregion; and forming a silicide layer on the p-doped silicon layer or thep-doped SiGe layer in the source/drain region.

According to another aspect of the present invention, a semiconductordevice is provided. The device includes: a silicon substrate having asource/drain region; a local oxide layer formed on the siliconsubstrate; at least one SiGe fin formed on the silicon substrate,wherein the at least one SiGe fin includes an extension region; a gatestructure formed on the local oxide layer and the extension region ofthe at least one SiGe fin, wherein the gate structure is encapsulated bysidewall spacer layers and a gate hard mask, the at least one SiGe finextends vertically from the silicon substrate through the sidewallspacer; a n-doped silicon layer grown on the silicon substrate, whereinthe n-doped silicon layer is grown in the source/drain region; a p-dopedsilicon layer or p-doped SiGe layer grown on the n-doped silicon layer,wherein the p-doped silicon layer or p-doped SiGe layer is grown in thesource/drain region; and a silicide layer formed on the p-doped siliconlayer or the p-doped SiGe layer, wherein the silicide layer is formed inthe source/drain region.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a three dimensional isometric view of a bulk FinFET with aSiGe pFET fin that can be found in the prior art.

FIG. 2 is a three dimensional isometric view of a semiconductor devicewith SiGe pFET fins, according to an embodiment of the presentinvention, after the SiGe fins are recessed.

FIG. 3 is a three dimensional isometric view of a semiconductor devicewith SiGe pFET fins, according to an embodiment of the presentinvention, after the local oxide layer in the source/drain region isrecessed.

FIG. 4 is a three dimensional isometric view of a semiconductor devicewith SiGe pFET fins, according to an embodiment of the presentinvention, after a n-doped silicon layer is grown on the siliconsubstrate layer in the source/drain region.

FIG. 5 is a three dimensional isometric view of a semiconductor devicewith SiGe pFET fins, according to an embodiment of the presentinvention, after a p-doped layer is grown on the n-doped silicon layer.

FIG. 6 is a three dimensional isometric view of a semiconductor devicewith SiGe pFET fins, according to an embodiment of the presentinvention, after a silicide layer is formed on the p-doped layer.

FIG. 7 is a flowchart depicting a method for fabricating a semiconductordevice with SiGe pFET fins, according to an embodiment of the invention.

DETAILED DESCRIPTION

With continued device pitch scaling, access resistance is becoming abarrier to improving device performance. In this scenario, achievingjunction overlap and reducing extension resistance are crucial toimproving device performance. According to an embodiment of the presentinvention, a method for forming extensions in a SiGe FinFET structurewith abrupt junctions is provided. The resulting structure has improvedshort-channel control and lower access resistance.

According to an embodiment of the present invention, FIGS. 1-6 depictthree dimensional isometric views of a semiconductor device after theoperations of a method for fabricating the device, as depicted in FIG.7, are performed. However, the present invention can be implemented invarious manners, and thus should not be to be limited to the embodimentsdisclosed herein.

SiGe fins for pFETs are the proposed solution for pFETs in 10 nm andbeyond. Short-channel control of the p-type doping profile, usuallyboron, and junctions at those small dimensions are very challenging,since diffusion of boron in SiGe is negligible. In a pFET, doping of thesilicon includes a hole being formed due to the absence of an electronin a covalent bond state. The missing electron would normally be part ofthe bond between one silicon atom and its neighbor. Doping can beaccomplished with a p-type impurity, for example, boron, being added tothe silicon. The junction between the p-type and n-type doped regions(referred to as a p-n junction) permits the silicon to function as asemiconductor device.

According to an embodiment of the present invention, the semiconductordevice includes a SiGe fin under the gate structure and silicon or SiGein the source/drain regions. A conventional fabrication process of pFETFinFET with a SiGe fin under the gate structure and silicon in thesource/drain region leaves devices underlapped because of low diffusionof boron in SiGe. This leads leaves the fins substantially undoped,causing the access resistance to be very high. SiGe fins are desired inFinFETs for their lower threshold voltage, higher hole mobility, andbetter device performance. Underlapped devices are structures where theextension doping is lower (typically lower than 1e¹⁹/cm³) at thegate-edge. Underlapped devices are not desirable because thedevice-to-device or bulk wafer-to-wafer variability degrades in additionto the higher access resistance. A desired structure has a junction insemiconductor under the spacer region of the fin, leading to overlapped,abrupt extension junctions with low access resistance. Overlappeddevices are the ones where extension doping is 1e¹⁹/cm³ or higher at thegate edge. According to an embodiment of the present invention, anoverlapped semiconductor device is formed with an abrupt junction at thegate edge. Additionally, the fins in the source drain regions can bedoped, which can help in reducing the access resistance of the MOSFETfurther.

Referring to FIGS. 1 & 7, FinFET device 100 with SiGe pFET fins 125 aand 125 b is provided in operation S10. FinFET device 100 includes abulk silicon substrate layer 105, a local oxide layer 110, and asource/drain region 115. FinFET device 100 further includes SiGe fins125 a and 125 b, gate structure 120, spacer layer 130, and gate hardmask 135. As depicted in FIG. 1, SiGe fins 125 a and 125 b are formed onbulk silicon substrate layer 105. Local oxide layer 110 is also formedon the portions of bulk silicon substrate layer 105 adjacent to SiGefins 125 a and 125 b. Gate structure 120 is formed on local oxide layer110 and SiGe fins 125 a and 125 b so that SiGe fins 125 a and 125 b areunder gate structure 120 and extend to a source/drain region on theother side of gate structure 120 (not shown). Spacer layer 130 is formedon the sides of gate structure 120. Spacer layer 130 can be made ofsilicon oxide or silicon nitride or silicon oxynitride or a combinationthereof. Gate hard mask 135 is formed on top of gate structure 120. Gatehard mask 135 can be made of the same or different material as spacerlayer 130. If the same material is used for gate hard mask 135 andspacer layer 130, then the density of the gate hard mask can bedifferent than that of the spacer layer.

Gate structure 120 can also include a thin layer of gate dielectric (notshown). The gate dielectric can be a layer of oxide or nitride or it canbe a layer of a high dielectric constant (high-k) material including adielectric metal oxide and having a dielectric constant that is greaterthan the dielectric constant of silicon nitride of 7.5. The high-kdielectric layer can be formed by methods well known in the artincluding, for example, chemical vapor deposition (CVD), atomic layerdeposition (ALD), molecular beam deposition (MBD), pulsed laserdeposition (PLD), liquid source misted chemical deposition (LSMCD), etc.The dielectric metal oxide includes a metal and oxygen, and optionallynitrogen and/or silicon. Exemplary high-k dielectric materials includeHfO₂, ZrO₂, La₂O₃, Al₂O₃, TiO₂, SrTiO₃, LaAlO₃, Y₂O₃, HfO_(x)N_(y),ZrO_(x)N_(y), La₂O_(x)N_(y), Al₂O_(x)N_(y), TiO_(x)N_(y),SrTiO_(x)N_(y), LaAlO_(x)N_(y), Y₂O_(x)N_(y), a silicate thereof, and analloy thereof. Each value of x is independently from 0.5 to 3 and eachvalue of y is independently from 0 to 2. The thickness of the high-kdielectric layer can be from 1 nm to 10 nm, and more preferably fromabout 1.5 nm to about 3 nm. The high-k dielectric layer can have aneffective oxide thickness (EOT) on the order of, or less than, about 1nm. A gate electrode can be deposited directly on the surface of thehigh-k dielectric layer by, for example, chemical vapor deposition(CVD), physical vapor deposition (PVD), or atomic layer deposition(ALD).

Source/drain region 115 can be doped by ion implantation. This isaccomplished by merging source/drain regions 115 with an epitaxy layerand doping this epitaxy layer in-situ, or by combining the in-situ dopedepitaxy process with single or multiple ion implantations. In anepitaxial growth process, a semiconductor material can be grown on thesurface of another semiconductor material. Both semiconductor materialshave the same crystalline characteristics. Epitaxy can include theformation of a layer on a crystalline surface, with the layer having thesame symmetry and crystalline orientation as the crystalline surface. Asingle crystal lattice structure often carries across an interface. Inepitaxy, a single crystal or monocrystalline material forms a platformonto which another single crystal material with matching crystallinecharacteristics is deposited by one of several techniques. Examples ofsuch processes include: ultrahigh vacuum chemical vapor deposition(UHVCVD), rapid thermal chemical vapor deposition (RTCVD), metal-organicchemical vapor deposition (MOCVD), low-pressure chemical vapordeposition (LPCVD), limited reaction processing chemical vapordeposition (LRPCVD), plasma-enhanced chemical vapor deposition (PECVD),and molecular beam epitaxy (MBE).

The terms “p-doped silicon” or “p-doped SiGe,” as used herein, refer tosilicon or SiGe having substitutional acceptor atoms located therein,respectively. The substitutional acceptor atoms and the silicon or SiGeatoms form a silicon-dopant or SiGe-dopant alloy, which is asemiconductor material. The dopant content of the epitaxial grownp-doped layers range from 0.02% to 0.1%, by atomic weight %. As in theembodiment depicted in FIG. 1, examples of acceptor dopants includeBoron (B), Difluoroborane (BF₂), and Indium (In), and the dopantconcentration can be on the order of about 10¹⁹ atoms/cm³, such as adopant concentration of about 1×10¹⁹ to 5×10²⁰ atoms/cm³, as examples ofp-doped Si or p-doped SiGe dopants and concentrations.

Ion implantation can cause fin damage and incomplete or norecrystallization upon annealing. This leads to defects and incompleteactivation of the dopant, which in turn leads to higher leakage andhigher access resistance, respectively. To circumvent this problem, animplant-free approach can be exercised. After depositing the in-situp-doped Si or p-doped SiGe epitaxy layer, the wafer can be annealed fordiffusion and activation of the dopant to allow for formation ofsource/drain region 115 and the extension junctions of SiGe fins 125 aand 125 b. Extension junctions (not shown) are part of SiGe Fins 125 aand 125 b and are formed under spacer layer 130 and gate structure 120.

Referring to FIGS. 2 & 7, FinFET device 100 is depicted after operationS20 is completed. In operation S20, SiGe fins 125 a and 125 b arerecessed to spacer layer 130 and to bulk silicon substrate layer 105.This creates an exposed region of bulk silicon substrate layer 105. Adry plasma etch process, such as reactive ion etch (RIE) process, is onesuitable technique to recess the fins using an etch chemistry that isselective to Si (e.g., CF₄), or similar industry process. As depicted inFIG. 2, SiGe fins 125 a and 125 b are recessed to spacer layer 130 thatencapsulates the sides of gate structure 120 forming a flat verticalwall including both SiGe fins 125 and spacer layer 130. SiGe fins 125 aand 125 b form an abrupt junction on bulk silicon substrate layer 105 inthe silicon component of the fins. After SiGe fins 125 a and 125 b arerecessed, a canal-like region is formed in local oxide layer 110. Bulksilicon substrate layer 105 is exposed by these canal-like regions.

The recessed SiGe fins 125 a and 125 b and bulk silicon substrate layer105 form abrupt junctions upon source/drain epitaxy formation, which isdesirable for improved short-channel control. Abrupt junctions reduceboth the drain-induced barrier lowering (DIBL), and the sub-thresholdslope (SSsat) of the device both of which lead to improved drive currentand performance.

Referring to FIGS. 3 & 7, FinFET device 100 is shown following thecompletion of operation S30. In operation S30, local oxide layer 110 insource/drain region 115 is recessed to spacer layer 130. This forms aflat, vertical wall, which includes SiGe fins 125 a and 125 b, spacerlayer 130, and local oxide layer 110, as shown in FIG. 3. A dry plasmaetch process, such as a reactive ion etch (RIE), is one suitabletechnique to recess local oxide layer 110 using an etch chemistry thatis selective to Si (e.g., CF₄), or similar industry process. Recessinglocal oxide top layer 110 leaves bulk silicon substrate layer 105exposed in source/drain region 115.

Referring to FIGS. 4 & 7, FinFET device 100 is depicted after operationS40 is performed. In operation S40, an n-doped silicon layer 140 isgrown on top of bulk silicon substrate layer 105 in source/drain region115. N-doped silicon layer 140 is formed directly on top of bulk siliconsubstrate layer 105, creating a contact between the bottom side ofn-doped silicon layer 140 and the top side of bulk silicon substratelayer 105, as depicted in FIG. 4. N-doped silicon layer 140 isolatessource/drain region 115 and helps prevent excessive leakage during thedevice-off state in short channel lengths devices. Punch through occursbetween the source and drain of a device when parasitic leakage pathoccurs deep in the transistor body that is poorly controlled by thegate. This phenomenon is undesirable as it increases the subthresholdleakage there by increasing the stand-by power consumption for the sameperformance. N-doped silicon layer 140 can be grown to ˜10-30 nm thickand have low to mid-e18/cm³ n-type doping level. The doping level can bedone by either using arsenic, phosphorus, or any method known in theart.

Still referring to FIGS. 4 & 7, after n-doped silicon layer 140 has beengrown bulk silicon substrate layer 105, n-doped silicon is etched fromthe sidewalls of SiGe fins 125 a and 125 b. Further etching occurs undergate structure 120 to better form the extensions under spacer layer 135.A hydrochloric acid (HCl) etch process can be used to etch the sidewallsof SiGe fins 125 a and 125 b and under gate structure 120. Thefabricator can then diffuse and activate the dopant in the region underthe spacer to ensure a well overlapped junction with low extensionresistance.

Referring to FIGS. 5 & 7, FinFET device 100 is shown following afteroperation S50 has been performed. In operation S50, p-doped siliconlayer 145 is grown on n-doped silicon layer 140, as shown in FIG. 5. Asan alternative to growing a p-doped silicon layer, a p-doped SiGe layercan be grown on n-doped silicon layer 140. P-doped silicon layer 145 isformed directly on top of n-doped silicon layer 140. A contact is formedbetween the bottom side of p-doped silicon layer 145 and the top side ofn-doped silicon layer 140. Doping of n-doped silicon layer 140 must bedone prior to p-doped silicon layer 145 being grown. P-doped siliconlayer 145 is doped typically using boron to a 2-4e²⁰/cm³ boron dopinglevel. With trisilane or tetrasilane, high boron doping level at lowtemperatures (˜550° C.) can be achieved. Low epitaxy growth temperatureis desirable to maintain SiGe fin strain.

Referring to FIGS. 6 & 7, FinFET device 100 is shown following thecompletion of operation S60. In operation S60, silicide layer 150 isformed on p-doped silicon layer 145, creating a contact between thebottom side of silicide layer 150 and the top side of p-doped siliconlayer 145, as depicted in FIG. 6. Silicide layer 150 can also be formedon top of epitaxially grown n-situ boron doped SiGe on top of p-doped Sisource/drain regions. If an in-situ doped SiGe layer is deposited beforethe silicide process, additional sourced/drain recess can be necessary.This additional recess can be performed by RIE.

The resulting structure includes a pFET FinFET with SiGe fins 125 a and125 b formed under gate structure 120. Source/drain region 115 of theFinFET includes n-doped silicon layer 145 grown on bulk siliconsubstrate layer 105, p-doped silicon layer 145 grown on n-doped siliconlayer 140, and silicide layer 150 formed on p-doped silicon layer 145.As an alternative, a p-doped SiGe layer can be formed instead of p-dopedsilicon layer 145.

The description of the various embodiments of the present invention havebeen presented for purposes of illustration, but are not intended to beexhaustive or limited to the embodiments disclosed. Many modificationsand variations will be apparent to those of ordinary skill in the artwithout departing from the scope and spirit of the describedembodiments. The terminology used herein was chosen to best explain theprinciples of the embodiments, the practical application or technicalimprovement over technologies found in the marketplace, or to enableothers of ordinary skill in the art to understand the embodimentsdisclosed herein.

What is claimed is:
 1. A method for fabricating a semiconductor device,the method comprising: providing a FinFET having a source/drain regionand at least one SiGe fin, wherein the at least one SiGe fin hassidewalls and is formed on a silicon substrate layer, a local oxidelayer is formed on the silicon substrate layer, a gate structure isformed on the at least one SiGe fin and the local oxide layer, the gatestructure is encapsulated by a gate hard mask and a plurality ofsidewall spacer layers; recessing the at least one SiGe fin in thesource/drain region, wherein the at least one SiGe fin is recessed tothe plurality of sidewall spacer layers and the silicon substrate layer;recessing the local oxide layer in the source/drain region, wherein thelocal oxide layer is recessed to the plurality of sidewall spacer layersand the silicon substrate layer; growing an n-doped silicon layer on thesilicon substrate layer in the source/drain region; growing either ap-doped silicon layer or a p-doped SiGe layer on the n-doped siliconlayer in the source/drain region; and forming a silicide layer on thep-doped silicon layer or the p-doped SiGe layer in the source/drainregion.
 2. The method according to claim 1, wherein growing an n-dopedsilicon layer on the silicon substrate layer in the source/drain regionfurther comprises: etching the n-doped silicon layer from the at leastone SiGe fin sidewalls; and etching under the plurality of sidewallspacer layers and the gate structure.
 3. The method according to claim1, wherein recessing the at least one SiGe fin in the source/drainregion exposes a portion of the silicon substrate layer in thesource/drain region.
 4. The method according to claim 1, whereinrecessing the at least one SiGe fin in the source/drain region furthercomprises growing an epitaxial layer to form an abrupt junction.
 5. Themethod according to claim 1, wherein recessing the local oxide layer inthe source/drain region exposes the silicon substrate layer in thesource/drain region.
 6. The method according to claim 2, wherein etchingthe n-doped silicon layer comprises a HCl etch.
 7. The method accordingto claim 1, wherein the n-doped silicon layer comprises a low tomid-e18/cm³ n-type doping level.
 8. The method according to claim 1,wherein the n-doped silicon layer comprises a thickness between 10-30nm.
 9. The method according to claim 1, wherein the p-doped siliconlayer or the p-doped SiGe layer comprises a 2-4e²⁰/cm³ boron dopinglevel.
 10. The method according to claim 1, wherein the p-doped siliconlayer or p-doped SiGe layer comprises a thickness between 40-50 nm. 11.The method according to claim 1, wherein the p-doped silicon layer orp-doped SiGe layer is grown using tetrasilane or trisilane.
 12. Asemiconductor device comprising: a silicon substrate layer having asource/drain region; a local oxide layer formed on the silicon substratelayer; at least one SiGe fin formed on the silicon substrate layer,wherein the at least one SiGe fin includes an extension region; a gatestructure formed on the local oxide layer and the extension region ofthe at least one SiGe fin, wherein the gate structure is encapsulated bya plurality of sidewall spacer layers and a gate hard mask, the at leastone SiGe fin extends vertically from the silicon substrate layer throughthe plurality of sidewall spacer layers; an n-doped silicon layer grownon the silicon substrate layer, wherein the n-doped silicon layer isgrown in the source/drain region; a p-doped silicon layer or p-dopedSiGe layer grown on the n-doped silicon layer, wherein the p-dopedsilicon layer or p-doped SiGe layer is grown in the source/drain region;and a silicide layer formed on the p-doped silicon layer or the p-dopedSiGe layer, wherein the silicide layer is formed in the source/drainregion.
 13. The semiconductor device according to claim 11, wherein theat least one SiGe fin forms an abrupt junction.
 14. The semiconductordevice according to claim 11, wherein the n-doped silicon layercomprises a low to mid-e¹⁸/cm³ n-type doping level.
 15. Thesemiconductor device according to claim 11, wherein the n-doped siliconlayer comprises a thickness between 10-30 nm.
 16. The semiconductordevice according to claim 11, wherein the p-doped silicon layer or thep-doped SiGe layer comprises a 2-4e²⁰/cm³ boron doping level.
 17. Thesemiconductor device according to claim 11, wherein the p-doped siliconlayer or p-doped SiGe layer comprises a thickness between 40-50 nm. 18.The semiconductor device according to claim 11, wherein the p-dopedsilicon layer or p-doped SiGe layer is grown using tetrasilane ortrisilane.